As the general decline in traditional energy resources has been experienced by industry, as well as the consuming public, a switch to the conversion of less desirable energy resources and the production of synthetic fuels and gases have occurred. Various processes which have been known in the prior art, but have not been practiced on a commercial scale because of previous unfavorable economics, are now beginning to be of interest to the energy industry as potential viable sources of consumable energy which can replace dwindling petroleum reserves. Such synthetic fuel production processes include oil shale retorting, coal gasification, and oxygen fireflooding. In addition, as easily recoverable petroleum reserves are used up, secondary and tertiary recovery systems for residual petroleum reserves are being utilized such as enhanced oil recovery using pressurized carbon dioxide gas.
Many of the synthetic fuel production processes as well as secondary and tertiary recovery operations utilize large quantities of inert gas as moderators, diluents and pressure maintaining phases. Various gases have been utilized for such processes including nitrogen and carbon dioxide.
In oil shale retorting, both surface and in-situ, a moderating gas is essential to the controlled combustion of the oil shale mineral. Oil shale is a sedimentary geologic mineral formation generally found in the western states of the United States. Oil shale contains a hydrocarbonaceous component which is called kerogen. In oil shale retorting, the object is to heat the kerogen until it is volatilized for successful removal as a synthetic fuel consisting of a gas phase and a liquid phase. During retorting, an oxidant gas and a moderating gas are forced through the retorting oil shale in order to burn a small percentage of the kerogen to provide heat for the volatilization of the remainder of the kerogen for recovery as a fuel. As combustion occurs in the retorting of oil shale, large quantities of carbon dioxide off-gases are formed which contain hydrocarbons and sulfur-containing gases. A problem exists in the disposal of these off-gases both to avoid environmental problems with sulfur components and to improve the economics of the presently very expensive oil shale retorting operation.
In another synthetic fuel production process, coal gasification, moderators are typically required for inclusion with the coal combusting oxidant gas. Steam has generally been utilized as a coal gasification moderator. Carbon dioxide has been considered as an alternate moderator. Again, in coal gasification the off-gas from the process includes fuel components as well as non-fuel components comprising carbon dioxide and sulfur-containing gases. In order to meet environmental and economic goals, these gases must be utilized or processed for disposal or further use.
In oxygen fireflooding, an oxidant gas is used to combust a petroleum formation in-situ. Such formations in which oxygen fireflooding is utilized generally do not naturally produce due to the lack of natural in-situ pressure, high viscosity of the petroleum in the formation or unfavorable formation structure. An oxidant gas is pumped into the petroleum formation in an injection well to spontaneously combust the formation or to sustain an artifically initiated combustion. Such combustion heats the petroleum and lowers its viscosity which allows the petroleum to be recovered from a producing well which is used in tandem with the injection well. During the petroleum combustion, a significant amount of carbon dioxide is produced and recovered with the produced petroleum. Again, this process would benefit economically from the utilization of the significant carbon dioxide by-product of the petroleum recovery in oxygen fireflooding.
In carbon dioxide miscible flood enhanced oil recovery operations, high pressure carbon dioxide is injected into a partially depleted oil reservoir. The carbon dioxide serves to extract and displace the residual oil to a production well that discharges carbon dioxide and recovered oil to the surface at reduced pressure. The oil product liquid phase is separated from the carbon dioxide and the hydrocarbon gas phase. The gas can be processed to recover the oil from the by-product gases. Again, the economics of the recovery process would benefit from the utilization of the carbon dioxide-containing gas phase.
In the above-identified synthetic fuel production processes, each process would benefit from the further utilization of carbon dioxide, which is a significant by-product, by either recycling the carbon dioxide with the sulfur-containing gases as a diluent for the combustion process or for the recovery of the carbon dioxide for use at off-site locations such as pipelining to enhanced oil recovery operations.
Various prior art processes have been developed for the recycling of such off-gases from synthetic fuel production processes, such as coal gasification and oil shale retorting, as well as recovery operations, such as oxygen fireflooding and enhanced oil recovery operations. These prior art processes generally suffer from high energy consumption and a complexity of process apparatus which requires high capital expenditure.
U.S. Pat. No. 2,886,405 discloses a process for the separation of carbon dioxide and hydrogen sulfide from gas mixtures utilizing a chemical absorbent solvent such as hot potassium carbonate. As is typical in chemical solvents, the enriched solvent is regenerated by a boiling and steam stripping operation which is energy intensive.
U.S. Pat. No. 4,014,575 describes a process for the recycling of off-gases from oil shale retorting through spent oil shale beds in order to deposit the sulfur compounds from the off-gas onto the particles of the oil shale bed. This can be done in conjunction with the water scrubbing of the off-gases in a Venturi scrubber.
In U.S. Pat. No. 4,117,886, a method is disclosed which utilizes the scrubbing of off-gases from oil shale retorting with water containing basic components. The acid-containing off-gas from an operating oil shale retort is contacted with this basic component-containing water. The basic pH water neutralizes the acid off-gases and the latter can be recycled for retorting or burned if sufficient BTU energy can be derived.
In U.S. Pat. No. 4,158,467, a process for the recycling of oil shale retort off-gases is disclosed wherein the hot potassium carbonate solvent of U.S. Pat. No. 2,886,405, mentioned above, is utilized. As stated before, the utilization of chemical absorbent solvents in such an operation is energy intensive due to the complexity of regenerating such solvents for reuse. Additionally, the chemical absorption process is essentially nonselective between sulfur compounds and carbon dioxide.
The removal of acid gas components from gas streams is set forth in U.S. Pat. No. 4,169,133 wherein carbon dioxide acid gas components are frozen out of a main gas stream. A process wherein a solid product is produced from a gas clean-up operation is not conducive to recycling or continuous operation.
In U.S. Pat. No. 4,169,506, the scrubbing of offgases from in-situ retorting of oil shale is set forth. The scrubbing utilizes caustic soda in conjunction with a deoiling process. In this instance, the scrubbed sulfur components are passed to a claus plant for refinement to elemental sulfur.
In U.S. Pat. No. 4,270,937, a process is disclosed for the separate removal of sulfides and carbon dioxide from a coal gasification gas stream. Externally supplied refrigeration is used to operate a complex solid/liquid absorbent stream in a process which operates on carbon dioxide streams containing 55% carbon dioxide.
The processes of the prior art for the disposal or recycle of carbon dioxide diluent gases to a synthetic fuel production process or for the utilization of such gas from an enhanced oil recovery operation have been deficient for several reasons, including; the energy intensive nature of the recovery operation, the regeneration requirements of chemical solvents, the necessity for large quantities of water, which in areas may not be available and the uneconomical separation and recycle of pressurizing gases for enhanced oil recovery operations. In addition, many of the prior art processes have failed to recover potential low BTU fuel components from the off-gases from such operations.
The present invention overcomes these disadvantages by providing a low energy, low temperature system for the recovery of carbon dioxide and acid gases for potential recycle or export, as well as the recovery of a low BTU fuel gas for export or plant use. The present invention achieves this recovery using a dual absorption column methanol scrub cycle.
With respect to oxygen fireflooding and enhanced oil recovery operations, the present invention can be used to extract and pipeline bulk carbon dioxide after additional sulfur removal or recycle of the carbon dioxide to the enhanced oil recovery operation from which it is produced.